Southern San Andreas Overdue for Large Quake

Below:

Next story in Science

The valley of Coachella in Southern California is known for
concerts held there every year, but new research has shed light
on a more dangerous kind of rocking that has occurred there over
the past millennium: quakes on a mysterious part of the San
Andreas fault.

The southernmost 60 miles (100 kilometers) of the
San Andreas fault is the only stretch of the fault that has
not ruptured in recorded history. This makes it hard to gauge
when the next earthquake might strike there or how damaging it
might be. This uncertainty is especially troubling considering a
major quake there
could severely damage Los Angeles, roughly 140 miles (225 km)
to the west.

To learn more about the southern part of the 800-mile-long (1,300
km) fault, scientists focused on an area of the city of Coachella
that was untouched by orchards or by canals, golf courses or
other developments, making evidence of past earthquakes
relatively easy to see.

The researchers investigated three trenches about 25 feet deep (8
meters) and up to roughly 1,300 feet (400 m) long. These exposed
how the earth along the fault was layered, helping the scientists
look at how the ground might have shifted due to past quakes.

Their analysis uncovered at least five and up to seven
major earthquakes at this part of the San Andreas during the
past 1,100 years, with one occurring about every 180 years. The
most recent quake occurred there around 1690. Because this
stretch of the fault has had more than 300 years to accumulate
stress, the researchers said it is likely to produce a major
quake in the next few decades.

"I would tell people living in Southern California that a major
San Andreas fault earthquake will very likely happen during their
lifetimes and that they need to be as prepared as possible,"
researcher Belle Philibosian, a geologist at the California
Institute of Technology, told OurAmazingPlanet.

Of course, scientists can't say exactly when this or any other
fault might rupture, but Philibosian emphasized the need for
preparedness.

"The exact timing of large earthquakes appears to be inherently
unpredictable — at least, all scientific efforts have so far
failed — so the way to deal with earthquakes is through building
codes and emergency planning," she added. "People should, if
possible, have their houses assessed for earthquake risk and have
them retrofitted if need be. It's also very important to have a
cache of emergency supplies."

For researchers, the next step is to see how much the fault
slipped during each of these past earthquakes, to figure out the
quakes' size and extent, Philibosian said.

Philibosian and her colleagues Thomas Fumal and Ray Weldon
detailed their findings in the February issue of the Bulletin of
the Seismological Society of America.